Shelby County, TN does border Mississippi, so its just possible that he was living there when he enlisted in a Mississippi unit. I'd be interested to see the 1860 Census records possibly showing where he and also where Linnie Sloan lived. In all probability, her family lived in Shelby County. Considering her age, he probably married her there after the war.

As for the place of his enlistment, that might be found on pension forms if he applied for one after the war. You might also learn something from 1870 and subsequent Census records. In addition, you might be able to learn more from the West Tennessee Historical Society. (Marriage records, property records, obituaries, etc.).